High pressure cylindrical vessels are successfully utilized in different high pressure applications. In particular they are used in food High Pressure Processing (HPP) systems also known as Pascalization. Some HPP systems are based on monoblock pressure cylinders that cause concerns with limited life cycle resulting in dangerous fatigue failures posing even a threat of explosions. In order to increase the pressure capacity of cylindrical chambers a number of techniques have been utilized in order to produce favourable residual stresses, including multilayer construction and autofrettage.
Multilayer vessels are assembled so as to have an interference fit between respective cylinders. This results in compressive residual stresses in the inner cylinder and tensile residual stresses in the outer cylinder. The interference fit between layers may be accomplished by heating and then shrinking the outer cylinder having a bore diameter slightly smaller than that of the respective inner cylinder or by having matched tapers on the inside and outside surface of the outer and inner cylinders respectively and forcing the elements into each other by means of press. The resultant interface pressures, residual stresses, operating stresses and the pressure capacity are a function of numerous variables including number of cylinders, their relative strengths, stiffness and diameter ratios.
In autofrettage method, residual stresses are induced by subjecting a thick-walled cylinder to an internal pressure exceeding its yield pressure. That leads to a plastic deformation that initiates at the bore and, as the pressure increases, proceeds through the cylinder wall. After the internal pressure is released, due to elastic recovery, the material near the outside surface of the cylinder, which has been deformed the least amount, will attempt to return to its original diameter and the material near the bore, which has been deformed the most, will attempt to remain deformed. This results in a residual compressive stress at the bore and a residual tensile stress at the outside surface with a gradual transition through the wall thickness. Autofrettage has been utilized for many years in the manufacture of gun barrels and in recent years it has also been applied to pressure vessels designed to operate at very high pressures.
Both multilayer and autofrettage techniques may be combined together in order to provide higher pressure capacity of the vessel. In that case, the most inner cylinder made of a high strength metal is autofrettaged and combined with jacket of outer cylinders which may be made of lower strength steels.
Maximum equivalent stresses of any thick-walled cylinder subjected to internal pressure appear at radially inner surface of this cylinder (effect known as Lame Problem). The predominant stress component which is influencing vessel capacity is tangential (hoop) stress. The same component determines response also for multilayer and/or or autofrettaged constructions. Therefore a construction of a multilayer cylindrical vessel has been proposed having an intermediate cylindrical layer sandwiched between the inner cylinder and the outer cylinder, said intermediate layer formed of a number equiangularly distributed circular segments separated by gaps. Each segment of such a segmented layer is no longer restricted in circumferential direction, so that tangential stresses are eliminated and only radial stresses are transmitted between the inner cylinder and the outer cylinder. Obviously such a sectioned layer may not be the innermost layer of a pressure vessel since it would not provide a fluid tight surface of such a vessel. Nonetheless the inner cylinder supported by the segments can either be a thin liner preventing the pressure fluid from entering the spaces between the segments or thick cylinder forming a structural component of the pressure vessel.
Such a multilayer construction with a segmented intermediate cylindrical layer has higher resistance to fatigue and increases pressure capacity of the vessel. It is however problematic from fabrication and assembling point of view.
Another exemplary multilayer wall pressure vessel has been disclosed in the U.S. Pat. No. 3,488,160.
It has been the object of the present invention to provide a durable and cost-effective multilayer high pressure cylindrical vessel devoid of the drawbacks of the prior art solutions which would be simply to assemble and would feature a pressure capacity and fatigue life comparable to vessels provided with an angularly segmented intermediate layer.